Field of the Present Invention
The present invention relates to a method for operating a lithographic apparatus. The present invention further relates to methods of manufacturing devices using lithographic apparatus calibrated by such a method, and to data processing apparatuses and computer program products for implementing parts of such a method.
Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., comprising part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction.
In the lithographic process, particularly where mass production of the most sophisticated products is required, many components and subsystems must work together with optimum performance to ensure accurate formation of device features at the correct locations on a substrate. Subsystems can lose integrity for a number of reasons, and any processing of substrates that are defective as a result can be very costly indeed. A particular issue is contamination of the patterning device (reticle) and/or the support and clamping arrangements that support it, leading to deformation or displacement of the reticle. This can degrade patterning performance of the lithographic process as a whole. The degraded performance may be detected when patterned substrates are subjected to inspection. However, by that time, a number of substrates may have been processed, and the cost and risk associated with re-working them is substantial. On the other hand, any dedicated inspection or measurement of the patterning device for the purpose of detecting contamination would only reduce the throughput of the process in normal operation.